After departing on an IFR flight plan from ZZZ1 en route to ZZZ2, I experienced the onset of vertigo climbing through approximately 8100 ft MSL while in cruise climb to my assigned altitude of 9000 ft MSL. The onset of vertigo was followed rapidly by the development of an unusual flight attitude from which I recovered after establishing visual ground contact after the aircraft had exited IMC conditions at the base of the overcast. The flight was a night crash fire rescue equipment 135 cargo flight, and was my normal assigned route which I had been flying for a few months. Upon departure I entered IMC conditions after receiving my initial on course vector from TRACON as the aircraft climbed through approximately 1700 ft AGL, and experienced intermittent IMC conditions through approximately 4500 ft MSL and therefore solid IMC for the remainder of the climb until the onset of vertigo and subsequent unusual flight attitude. I did experience light-to-moderate chop turbulence throughout the duration of the flight. My first indication that something was not right occurred a few moments after being cleared direct to the VOR with instructions to climb and maintain 9000 ft MSL. As the aircraft climbed through 8100 ft MSL I noted a slight right bank angle (2.5 degrees) on the attitude indicator. I then applied slight coordinated left aileron and rudder to correct. However, the bank angle had now progressed through approximately 7 degrees on the attitude indicator and I immediately scanned the turn indicator needle and again input left correction. The next scan of the attitude indicator showed the bank had progressed through 30 degrees and was increasing. A descent had begun and the airspeed was increasing. From this point the severity of the unusual attitude developed rapidly as I attempted to regain positive control of the aircraft. I was however able to obtain the specific radar data from TRACON which tracked the incident flight. This data did clearly support my memory of the altitudes, bank angle, descent progression and general flight path of the aircraft throughout the event. When the aircraft exited the base of the overcast at approximately 5000 ft MSL it was in a vertical-to-slightly-inverted attitude. With visual ground contact I was immediately able to reestablish positive control. I then reconfirmed my current position with TRACON and informed them of the unusual attitude and requested radar vectors for the approach back to my departure airport. The duration of this loss of control event was 35 seconds from onset to recovery of positive control. No emergency was declared, there was no damage to the aircraft or cargo. The day after the above-referenced incident of vertigo and unusual flight attitude, I went to my primary physician for an examination. My physician informed me after the examination that I had 2 allergic conditions: 1) serous otitis -- a build up of fluid behind the tympanic membrane of the inner ear, and 2) allergic rhinitis -- a light build up of fluid and inflammation in the right nasal passage and cavity. I was unaware that I was affected by either of these conditions prior to this medical examination. My physician explained to me that serous otitis often does not have any painful symptoms and will sometimes manifest itself only as a sense of 'fullness in the ear' but becomes more serious the longer one has it as the chance of infection increases and one is exposed to higher altitudes in a non-pressurized environment.

Narrative: AFTER DEPARTING ON AN IFR FLT PLAN FROM ZZZ1 ENRTE TO ZZZ2, I EXPERIENCED THE ONSET OF VERTIGO CLBING THROUGH APPROX 8100 FT MSL WHILE IN CRUISE CLB TO MY ASSIGNED ALT OF 9000 FT MSL. THE ONSET OF VERTIGO WAS FOLLOWED RAPIDLY BY THE DEVELOPMENT OF AN UNUSUAL FLT ATTITUDE FROM WHICH I RECOVERED AFTER ESTABLISHING VISUAL GND CONTACT AFTER THE ACFT HAD EXITED IMC CONDITIONS AT THE BASE OF THE OVCST. THE FLT WAS A NIGHT CFR 135 CARGO FLT, AND WAS MY NORMAL ASSIGNED RTE WHICH I HAD BEEN FLYING FOR A FEW MONTHS. UPON DEP I ENTERED IMC CONDITIONS AFTER RECEIVING MY INITIAL ON COURSE VECTOR FROM TRACON AS THE ACFT CLBED THROUGH APPROX 1700 FT AGL, AND EXPERIENCED INTERMITTENT IMC CONDITIONS THROUGH APPROX 4500 FT MSL AND THEREFORE SOLID IMC FOR THE REMAINDER OF THE CLB UNTIL THE ONSET OF VERTIGO AND SUBSEQUENT UNUSUAL FLT ATTITUDE. I DID EXPERIENCE LIGHT-TO-MODERATE CHOP TURB THROUGHOUT THE DURATION OF THE FLT. MY FIRST INDICATION THAT SOMETHING WAS NOT RIGHT OCCURRED A FEW MOMENTS AFTER BEING CLRED DIRECT TO THE VOR WITH INSTRUCTIONS TO CLB AND MAINTAIN 9000 FT MSL. AS THE ACFT CLBED THROUGH 8100 FT MSL I NOTED A SLIGHT R BANK ANGLE (2.5 DEGS) ON THE ATTITUDE INDICATOR. I THEN APPLIED SLIGHT COORDINATED L AILERON AND RUDDER TO CORRECT. HOWEVER, THE BANK ANGLE HAD NOW PROGRESSED THROUGH APPROX 7 DEGS ON THE ATTITUDE INDICATOR AND I IMMEDIATELY SCANNED THE TURN INDICATOR NEEDLE AND AGAIN INPUT L CORRECTION. THE NEXT SCAN OF THE ATTITUDE INDICATOR SHOWED THE BANK HAD PROGRESSED THROUGH 30 DEGS AND WAS INCREASING. A DSCNT HAD BEGUN AND THE AIRSPD WAS INCREASING. FROM THIS POINT THE SEVERITY OF THE UNUSUAL ATTITUDE DEVELOPED RAPIDLY AS I ATTEMPTED TO REGAIN POSITIVE CTL OF THE ACFT. I WAS HOWEVER ABLE TO OBTAIN THE SPECIFIC RADAR DATA FROM TRACON WHICH TRACKED THE INCIDENT FLT. THIS DATA DID CLRLY SUPPORT MY MEMORY OF THE ALTS, BANK ANGLE, DSCNT PROGRESSION AND GENERAL FLT PATH OF THE ACFT THROUGHOUT THE EVENT. WHEN THE ACFT EXITED THE BASE OF THE OVCST AT APPROX 5000 FT MSL IT WAS IN A VERT-TO-SLIGHTLY-INVERTED ATTITUDE. WITH VISUAL GND CONTACT I WAS IMMEDIATELY ABLE TO REESTABLISH POSITIVE CTL. I THEN RECONFIRMED MY CURRENT POS WITH TRACON AND INFORMED THEM OF THE UNUSUAL ATTITUDE AND REQUESTED RADAR VECTORS FOR THE APCH BACK TO MY DEP ARPT. THE DURATION OF THIS LOSS OF CTL EVENT WAS 35 SECONDS FROM ONSET TO RECOVERY OF POSITIVE CTL. NO EMER WAS DECLARED, THERE WAS NO DAMAGE TO THE ACFT OR CARGO. THE DAY AFTER THE ABOVE-REFED INCIDENT OF VERTIGO AND UNUSUAL FLT ATTITUDE, I WENT TO MY PRIMARY PHYSICIAN FOR AN EXAMINATION. MY PHYSICIAN INFORMED ME AFTER THE EXAMINATION THAT I HAD 2 ALLERGIC CONDITIONS: 1) SEROUS OTITIS -- A BUILD UP OF FLUID BEHIND THE TYMPANIC MEMBRANE OF THE INNER EAR, AND 2) ALLERGIC RHINITIS -- A LIGHT BUILD UP OF FLUID AND INFLAMMATION IN THE R NASAL PASSAGE AND CAVITY. I WAS UNAWARE THAT I WAS AFFECTED BY EITHER OF THESE CONDITIONS PRIOR TO THIS MEDICAL EXAMINATION. MY PHYSICIAN EXPLAINED TO ME THAT SEROUS OTITIS OFTEN DOES NOT HAVE ANY PAINFUL SYMPTOMS AND WILL SOMETIMES MANIFEST ITSELF ONLY AS A SENSE OF 'FULLNESS IN THE EAR' BUT BECOMES MORE SERIOUS THE LONGER ONE HAS IT AS THE CHANCE OF INFECTION INCREASES AND ONE IS EXPOSED TO HIGHER ALTS IN A NON-PRESSURIZED ENVIRONMENT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.